ALMOST every extrasolar planet discovered so far has been detected indirectly by measuring the wobble it induces in its host star. Spotting a planet directly is a huge challenge because the star is at least 10 million times brighter and completely overwhelms any light from the planet. But a simple device called an optical vortex could help astronomers blot out the starlight and bring planets into focus.
So far, attempts to see planets directly have involved positioning a small opaque disc in a telescope so that it blocks out the star鈥檚 light. But light from the star can still bend round the edges of the disc, creating bright diffraction bands that swamp signs of the planet.
Now Grover Swartzlander, Gregory Foo and David Palacios from the University of Arizona in Tucson think that the starlight could be removed using a helical mask with a series of steps etched into a transparent material (see Diagram). Light travels more slowly through the mask than it does through air. And because some parts of the mask are thicker than others, it creates a phase difference between the light that travels through the various sections. When the phase difference between the thickest and the thinnest parts is exactly two full waves, something unexpected happens: light in the central core destructively interferes, creating an 鈥渙ptical vortex鈥� that has a dark core with light spinning out into a bright ring around it.
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The mask can block out the starlight, leaving a dark core that acts as a window through which a planet鈥檚 light can pass (Optics Letters, vol 30, p 3308). The vortex will work only with powerful telescopes capable of resolving the star from its planet, so that the light from the planet and star come in at different angles. In laboratory trials using lasers to mimic a star and planet, the vortex mask cut the 鈥渟tarlight鈥� by factors of between 100 and 1000 without blocking any of the planet鈥檚 light.
But there are limitations. The pitch of the helical mask determines the wavelength of light that it will block out. Essentially each mask works for only one colour. For the technique to be practical, Swartzlander says that the mask would have to work over a wider range of wavelengths. Also, the mask鈥檚 optical quality must be improved. 鈥淏ut these are the first experiments to demonstrate the basic idea,鈥� Swartzlander says. 鈥淭heoretically, our technique would make an arbitrarily dim planet visible.鈥�
鈥淭he mask blocks the starlight, leaving a dark core that acts as a window through which a planet鈥檚 light can pass鈥�
His team hopes the vortex might be useful for projects such as NASA鈥檚 Terrestrial Planet Finder (TPF). But Steve Kilston of Ball Aerospace and Technologies in Boulder, Colorado, a company contracted by NASA to work on the TPF design, says the mask鈥檚 single-colour results do not come close to demonstrating the performance needed to see the multicoloured light from a faint planet.
